The present invention relates to an optical multi/demultiplexer which is small in size and has a high performance of wavelength separability and which constitutes a wavelength division multiplex transmission system.
An optical multi/demultiplexer of the directional coupler type is constructed by utilizing the fact that the coupling characteristic between two waveguides exhibits a wavelength dependence. Such techniques are described in, for example, "FREQUENCY-SELECTIVE COUPLING IN PARALLEL DIELECTRIC WAVEGUIDES" by H. F. TAYLOR, OPTICS COMMUNICATIONS, Volume 8, number 4, pp. 421-425, August 1973, and Japanese Patent Application Laid-open No. 14205/1985. The wavelength dependence of the coupling characteristic is fundamentally divided into two classes; (i) one based on the difference of the propagation characteristics of the two waveguides, and (ii) the other based on the degree of coupling which varies depending upon wavelengths. With either class, however, only one stage is difficult of attaining a satisfactory inter-wavelength separation. As a method of enhancing characteristics, there has heretofore been known an example described in "Filter Characteristics of Codirectionally Coupled Waveguides with Weighted Coupling" by R. C. ALFERNESS et al., IEEE JOURNAL OF QUANTUM ELECTRONICS, VOL. QE-14, NO. 11, pp. 843-847, NOVEMBER 1978. As described in "Directional Coupler Type Optical Demultiplexing Device" by SASAKI et al. (' 78 General National Meeting of the Japan Society of Electronics and Communication Engineering, S6-2), a method has also been proposed in which an optical coupling/branching device is constructed by connecting optical couplers in many stages, thereby to improve characteristics. In this case, however, the device becomes large in size inevitably. Usually the length of one coupler is on the order of several mm, so that the device constructed of the multistage couplers by the use of the prior art becomes as large as several cm. Since the present-day micro-processing technology has progressed premising on the production of IC chips of several mm square, the application thereof to the device of the size of several cm is difficult. Even if the technology can be applied, the productivity will inevitably lower. Also in the aspects of device performance characteristics, the enlarged size of the device poses the problem that an insertion loss increases.